Various components in gas turbine engines utilise arrangements of through-holes in their walls for the purpose of establishing various kinds and configurations of cooling arrangements therein. Such components may include for example combustor cassettes, combustion chamber heat shields and barrels, turbine blades, guide vanes, exhaust ducts and so on, and they are often manufactured by conventional investment casting techniques. Typically such components include small, substantially straight through-holes, often a plurality or even a large number thereof, which extend through the thickness of a wall of the component in order to extract heat from the material of the component body typically by producing a uniform film of a cooling fluid, e.g. cooling air, on the hot outer surface of the component. One method of extracting a larger amount of heat from the material than might otherwise be possible is to use two separate but closely spaced walls instead of one only thereof. In one simple constructional form, the hotter (inner) wall may be manufactured as a separate cast tile component which is bolted onto the cooler (outer) wall, thereby enabling a high temperature-capable cast alloy to be used for the inner wall.
The use of such small through-holes for film cooling can present difficulties in servicing, maintenance and/or repair procedures, as it is often necessary to be able to detect whether any—or any appreciable number—of the holes are blocked, e.g. as a result of the presence or build-up of combustion or other deposits such as dirt, pollution or environmental residues during extended use of the engine, or even as a result of remnant material left over from the manufacture of the component itself. Hole blockages can pose significant mechanical and safety risks, since a hole that is blocked cannot pass therethrough a required flow of cooling fluid, which may result—especially if several or many holes are blocked simultaneously—in the component overheating, or even ultimately prematurely failing. There is therefore a general need in the art to be able to routinely and efficiently inspect or test components for determining whether—and/or to what extent—any such through-holes are blocked.
In the above-mentioned simple dual-walled cooling arrangement, since the two walls are separate components and because they use straight holes, the task of inspecting the holes for blockages can be relatively straightforward. For example, the two wall components may be detached from one another, and a simple visual inspection procedure used, in which each component in turn is placed in front of a strong visible light source, and the line-of-sight direction through each hole is used to visually inspect it for possible blockage: if a hole is clear (i.e. not blocked), then light gets through to the observer and that can be readily seen and noted, whereas if a hole is blocked (or partially blocked), then no (or only limited) light gets through to the observer. Again, that can be readily seen by the observer's eye and noted, thereby enabling appropriate action to be taken to unblock the identified hole.
The above relatively simple known procedure is illustrated schematically in FIG. 1 of the accompanying drawings. In FIG. 1(a) the two wall components 2 (inner), 4 (outer) are shown mounted in juxtaposition, as they are in use, with their respective cooling through-holes labelled as 12, 14. In FIG. 1(b) the two wall components 2, 4 have been separated, thereby enabling each component in turn to be inspected by illuminating each one with a strong visible light source 20. The line-of-sight direction (indicated by the dotted line) through each hole 12, 14 allows the observer 30 to observe each hole 12, 14 to determine whether—and/or to what relative extent—light is passing through it. This therefore constitutes a direct visual indicator of whether the respective hole 12, 14 is clear or whether it is blocked, or perhaps partially blocked, and as a result of that determination appropriate remedial action to unblock the identified hole can be taken.
In some more modern gas turbine engines, certain through hole-cooled components such as those mentioned above are increasingly being manufactured using additive layer manufacturing (ALM) methods, which involve building up the component walls and other features incrementally from a laser-fusible powder of the required component material. Such ALM methods can be particularly useful because they allow the creation of relatively complex designs of components in a relatively simple and cheap manner. This can for example include mechanically integrated designs of cooling arrangements comprising virtually any number and arrangement of through-holes in individual skins or walls of multi-walled or integral box-like components, which using ALM techniques may thus be formed essentially as unitary bodies. This may lead for instance to significant weight and cost advantages, because of the ability to reduce wall thicknesses compared with what is possible using conventional casting techniques, whilst maintaining structural integrity and strength of the component. Examples of such components include those comprising mechanically integrated impingement effusion cooling systems.
However, the use of ALM methods comes with new disadvantages with respect to being able to inspect such components for potential blockage of through-holes in the walls thereof using a conventional line-of-sight inspection technique using visible light as discussed above. This is because although ALM manufacturing tolerances allow accurate relative positioning between e.g. impingement and effusion holes, this is only possible by building both hot and cold walls of a multi-walled component as a single integrated piece, and in that case through-holes in each respective wall may often not be in alignment such as to permit use of the above line-of-sight inspection technique. Furthermore ALM methods also allow complex, non-straight through-hole shapes to be manufactured, which can be useful to employ in practice but which again may prevent the use of the above line-of-sight inspection technique because of the absence of a direct straight line light path through such a non-straight hole configuration. Moreover, in the case of unitary multi-walled components which define a void between a pair of walls, e.g. spaced apart “hot” and “cold” walls formed in a single piece with each wall containing its own arrangement of through-holes (perhaps even straight holes), the mere presence of the void may also often prevent use of the above line-of-sight inspection technique, since through-holes of the respective sets in the facing walls may often be arranged with their longitudinal axes non-coincident, in which case it is impossible for light to pass through both sets of holes simultaneously even if both holes of a given pair in the two walls are clear/unblocked.
Examples of the preceding arrangements are illustrated schematically in FIG. 2 of the accompanying drawings. In FIG. 2(a) the two wall components 2′ (inner), 4′ (outer) are shown as a unitarily formed integrated piece, with their respective cooling through-holes labelled as 12′, 14′. As often may be the case in practice, the respective pairs of through-holes 12′, 14′ are not in register, i.e. their respective longitudinal axes are not coincident, so light from the light source 20 is prevented from reaching an observer 30 even if the holes 12′, 14′ are clear/unblocked. On the other hand, FIG. 2(b) shows a complex hole arrangement in which a non-straight complex hole 12″ in the wall component 2″ cannot even itself transmit a straight beam of light from the light source 20 to the observer 30, owing to the non-straight configuration of the hole passage itself.
Thus, in many modern manufacturing techniques for engine components comprising various arrangements and configurations of one or more through-holes in at least one wall thereof, conventional line-of sight inspection techniques are impossible to use.
Hitherto in the published art there have been several proposals for inspection systems for testing engine components for partial or complete blockage of through-holes in walls thereof. These have included for example systems based on measuring flow rates of a test liquid pumped through selected through-holes in the component, or mapping infra-red signatures or hotspots produced by impinging jets of steam passing through the holes, or even positron emission tomography. Such known systems are generally complex, expensive and slow to implement on a large industrial scale, and they also fail to address at least some of the specific problems associated with modern component manufacturing methods, such as ALM, as discussed above.
It is therefore a primary object of the present invention to address the above shortcomings of the prior art.